Optical frequency averaging of light.

The use of averaging has long been known to reduce noise in statistically independent systems that exhibit similar levels of stochastic fluctuation. This concept of averaging is general and applies to a wide variety of physical and man-made phenomena such as particle motion, shot noise, atomic clock stability, measurement uncertainty reduction, and methods of signal processing. Despite its prevalence in use for reducing statistical uncertainty, such averaging techniques so far remain comparatively undeveloped for application to light. We demonstrate here a method for averaging the frequency uncertainty of identical laser systems as a means to narrow the spectral linewidth of the resulting radiation. We experimentally achieve a reduction of frequency fluctuations from 40 Hz to 28 Hz by averaging two separate laser systems each locked to a fiber resonator. Only a single seed laser is necessary here as acousto-optic modulation is used to enable independent control of the second path. This technique of frequency averaging provides an effective solution to overcome the linewidth constraints of a single laser alone, particularly when limited by fundamental noise sources such as thermal noise, irrespective of the spectral shape of noise.

Cooling of an integrated Brillouin laser below the thermal limit.

Photonically integrated resonators are promising as a platform for enabling ultranarrow linewidth lasers in a compact form factor. Owing to their small size, these integrated resonators suffer from thermal noise that limits the frequency stability of the optical mode to ∼100 kHz. Here, we demonstrate an integrated stimulated Brillouin scattering (SBS) laser based on a large mode-volume annulus resonator that realizes an ultranarrow thermal-noise-limited linewidth of 270 Hz. In practice, yet narrower linewidths are required before integrated lasers can be truly useful for applications such as optical atomic clocks, quantum computing, gravitational wave detection, and precision spectroscopy. To this end, we employ a thermorefractive noise suppression technique utilizing an auxiliary laser to reduce our SBS laser linewidth to 70 Hz. This demonstration showcases the possibility of stabilizing the thermal motion of even the narrowest linewidth chip lasers to below 100 Hz, thereby opening the door to making integrated microresonators practical for the most demanding future scientific endeavors.

Continuous real-time photoacoustic demodulation via field programmable gate array for dynamic imaging of zebrafish cardiac cycle.

A four dimensional data set of the cardiac cycle of a zebrafish embryo was acquired using postacquisition synchronization of real time photoacoustic b-scans. Utilizing an off-axis photoacoustic microscopy (OA-PAM) setup, we have expanded upon our previous work with OA-PAM to develop a system that can sustain 100 kHz line rates while demodulating the bipolar photoacoustic signal in real-time. Real-time processing was accomplished by quadrature demodulation on a Field Programmable Gate Array (FPGA) in line with the signal digitizer. Simulated data acquisition verified the system is capable of real-time processing up to a line rate of 1 MHz. Galvanometer-scanning of the excitation laser inside the focus of the ultrasonic transducer enables real data acquisition of a 200 by 200 by 200 pixel, volumetric data set across a 2 millimeter field of view at a rate of 2.5 Hz.